1. Field of the Invention
The present invention generally relates to charged-particle-beam exposure devices and charged-particle-beam exposure methods, and particularly relates to a charged-particle-beam exposure device and a charged-particle-beam exposure method which expose a wafer to a charged-particle beam to form a pattern on the wafer.
2. Description of the Related Art
In recent development of increased circuit density of integrated circuits, an exposure technique using a charged-particle beam such as an electron beam has been gradually replacing a conventional photolithography technique as a method of forming a pattern on a semiconductor wafer. An exposure technique using an electron beam includes a variable-rectangle exposure technique and a block exposure technique.
The block exposure technique uses an aperture mask having a plurality of apertures of various pattern shapes. An electron beam is directed to a selected pattern of one or more apertures, and passes through the apertures to form an exposure pattern the same as the selected pattern on the wafer. The aperture mask is provided with aperture patterns repeatedly used during an exposure process. The block exposure technique is particularly effective when most exposure patterns are comprised of a repetition of basic patterns such as in 1G-DRAM chips or 4G-DRAM chips. In this case, patterns which are not repeated on the wafer are formed by using an electron beam having a variable-rectangle shape.
FIG. 1 is a block diagram of an example of an electron-beam-exposure device of the block-exposure type of the related art.
In FIG.1, an electron-beam exposure device 100 includes an exposure-column unit 110 and a control unit 150. The exposure-column unit 110 includes an electron-beam generator 114 having a cathode 111, a grid 112, and an anode 113. The exposure-column unit 110 further includes a first slit 115 shaping the electron beam rectangular, a first lens 116 converging the shaped beam, and a slit deflector 117 deflecting a position of the shaped beam on a block mask 120 based on a deflection signal S1. The exposure-column unit 110 further includes second and third lenses 118 and 119 opposing each other, the block mask 120 mounted movably in a horizontal direction between the second and third lenses 118 and 119, and first-to-fourth deflectors 121 through 124 deflecting the beam between the second and third lenses 118 and 119 based on position information P1 through P4 to select one of a plurality of apertures provided through the block mask 120. The exposure-column unit 110 further includes a blanking 125 cutting off or passing the beam according to a blanking signal, a fourth lens 126 converging the beam, an aperture 127, a refocus coil 128, and a fifth lens 129. The exposure-column unit 110 further includes a dynamic focus coil 130, a dynamic stigmator coil 131, an objective lens 132 projecting the beam onto a wafer W, and a main deflector 133 and a sub-deflector 134 positioning the beam on the wafer according to exposure-position signals S2 and S3. The exposure-column unit 110 further includes a stage 135 carrying the wafer to move it in X-Y directions, and first-to-fourth alignment coils 136 through 139.
The control unit 150 includes memory media 151 comprising a disk or MT recorder for storing design data of integrated circuits, and a CPU 152 controlling the electron-beam exposure device. The control unit 150 further includes a data-management unit 153, an exposure-management unit 159, a mask-stage controlling unit 160, a main-deflector-deflection setting unit 161, and a stage controlling unit 162, all of which are connected via a data bus (i.e., VME bus). Exposure data includes main-deflector data and sub-deflector data, and is stored in a buffer memory 154 via the data-management unit 153 prior to the exposure process. The buffer memory 154 is used as a high-speed buffer for reading the exposure data, thereby negating an influence of low-speed data reading from the memory media 151.
The main-deflector data is set in the main-deflector-deflection setting unit 161 via the exposure-management unit 159. The exposure-position signal S2 is output after the deflection amount is calculated, and is provided to the main deflector 133 via a DAC/AMP 170. Then, the sub-deflection data for exposing a selected field is read from the data-management unit 153, and is sent to a sub-deflector-deflection setting unit 155. In the sub-deflector-deflection setting unit 155, the sub-deflection data is broken down into shot data by a pattern generating unit 156, and is corrected by a pattern-correction unit 157. These circuits operate in a pipeline according to a clock signal generated by a clock setting unit 158.
After the processing of the pattern-correction unit 157, the signal S1 for setting a slit size, mask-deflection signals P1 through P4 for determining a deflected position on the block mask 120 of the beam deflected according to the signal S1 after passing through the first slit 115, the signal S3 for determining a position on the wafer of the beam shaped by the block mask 120, and a signal S4 for correcting distortion and blurring of the beam are obtained. The signal S1, the mask-deflection signals P1 through P4, the signal S3, and the signal S4 are supplied to the exposure-column unit 110 via a DAC(digital-to-analog converter)/AMP(amplifier) 166, a DAC/AMP 167, a DAC/AMP 171, and a DAC/AMP 169. Also, the clock setting unit 158 provides a blanking controlling unit 165 with a B signal. A BLK signal for controlling the blanking operation from the blanking controlling unit 165 is supplied to the blanking 125 via an AMP 168.
An exposure position on the wafer is controlled by the stage controlling unit 162. In doing so, a coordinate position detected by a laser interferometer 163 is supplied to the stage controlling unit 162. Referring to the coordinate position, the stage controlling unit 162 moves the stage 135 by driving a motor 164.
In this manner, the control unit 150 controls the exposure-column unit 110 such that the electron beam emitted from the electron-beam generator 114 is shaped rectangular by the first slit 115, converged by the lenses 116 and 118, deflected by the mask deflectors 121 and 122, and directed to the block mask 120. The beam having passed through the block mask 120 passes through the blanking 125, is converged by the fourth lens 126, deflected to a center of a sub-field of about a 100-.mu.m square by the main deflector 133, and deflected within this sub-field by the sub deflector 134.
A portion of the block mask 120 for forming aperture patterns are made into a thin layer, and the aperture patterns are formed by etching. As a base for the block mask 120, a semiconductor plate such as a Si plate or a metal plate is used.
A mask (block mask) used in the block exposure is provided with a rectangle aperture used by the variable-rectangle exposure and with aperture patterns of various complex shapes. In order to achieve a reliable and accurate exposure by using such a mask, the mask must be accurate and free of defections. In particular, when the block exposure technique is applied to mass manufacturing, a reliability of the mask must be guaranteed, so that an inspection technique for the block mask should be established.
For a thorough inspection of the block mask, three different inspections should be conducted in the same manner as for a conventional photo mask, including inspection of a pattern scale, inspection of a pattern position, and inspection of a pattern surface. Minimum scales of block-mask patterns are 10 to 20 times as large as those of photo-mask patterns. Thus, a conventional photo-mask inspection device can be used for pattern-scale inspection and pattern-position inspection of the block masks. As for the pattern-surface inspection, a size of an intolerable mask deficiency is larger than that for the photo-mask inspection, so that a conventional pattern-deficiency inspection device can be used to conduct an appropriate pattern-surface inspection.
As described above, a preliminary inspection of a block mask is easy. After mounting a block mask preliminarily inspected to a block-exposure-type electron-beam-exposure device, however, it is necessary to constantly check the block mask as to mask deficiencies which may be caused in an operating environment of the device.
For example, in order to keep accurate patterns intact, attachment of dust to the mask should be avoided. In the case of a photo mask, such attachment of dust can be avoided by coating the mask with a transparent layer. In the case of the block mask, however, it is difficult to create so thin a layer as to be transparent to the electron beam, so that satisfactory prevention of dust attachment is hard to achieve in practice. Also, temperature of the mask increases through exposure to the electron beam during an exposure process, so that a risk of mask distortion and mask melting due to the heat cannot be excluded.
Accordingly, a preliminary inspection of a block mask is not sufficient since various factors exist in the exposure device to cause various mask deficiencies. Thus, a way to conduct a mask inspection inside the exposure device becomes necessary.
A method for conducting the mask inspection inside an exposure device includes a method in which an image of an electron beam passing through a mask is visually inspected (Proceedings of the 53rd Convention of Institute of Applied Physics, No.2, 1992). In this method, the electron beam is scanned over the mask to obtain a relation between a scan position of the electron beam and the amount of electrons passing through the mask, and a result obtained as a passing-electron image is inspected.
Since this method relies on a visual inspection to check the electron image, however, an increased complexity or an increased number of mask patterns results in too long an inspection time and less reliable inspection results. It is possible to automatically inspect the electron image by applying image processing, but it is difficult to establish a high-speed and reliable inspection algorithm. Also, the inspection by the method of this reference is carried out for the image of the electron beam passing through the mask pattern, and is not a direct inspection of the pattern projected onto the wafer. Therefore, this method is ineffective when exposure deficiencies are caused by some factors intervening between the mask and the wafer rather than caused by mask deficiencies themselves.
Accordingly, there is a need for a charged-particle-beam exposure device and a charged-particle-beam exposure method which allow mask-pattern inspection in order to prevent pattern-exposure deficiency on a wafer from developing due to mask deficiency or the like.
Moreover, the same problem of exposure-pattern deficiency caused by mask-pattern deficiency or the like is also observed in the variable-rectangle exposure method. This problem in the variable-rectangle exposure method will be described below.
In the variable-rectangle exposure method, an electron beam generated by an electron gun is shaped so as to have a cross section of a desired rectangle shape, and is exposed onto an exposure material such as a wafer or the like. For the creation of the desired rectangle shape, the electron beam is passed through two slits (rectangle apertures), so that the cross section of the electron beam is formed into a shape of the superimposition of the two slits. In this manner, a desired rectangle shape is provided for the cross section of the electron beam. As can be understood, relative positions of the two slits have to be maintained with precision.
There are cases, however, in which the shape of the superimposition is different from the desired pattern because the slits are at angle in a horizontal direction or in a vertical direction with respect to the appropriate position thereof, or in which the cross section of the electron beam is warped by the large amount of deflection for directing the electron beam to a slit. When rotation or distortion is developed in an exposure pattern, a deflector, a distortion correction coil, or the like provided in the electron-beam-exposure device is used for effecting necessary correction.
However, detection of such exposure pattern deficiency is not easy. Generally, an exposure is carried out for a resist layer applied onto a wafer, and a developed pattern thereof is subjected to a visual inspection using an electron microscope or the like in order to check a rotation direction, a rotation amount, a distortion direction, a distortion amount, etc., of the developed pattern. Based on the inspection of the developed pattern, correction amounts are estimated by educated trial and error, and are supplied to the deflector, the correction coil, etc. Then, an exposure, a development, and an inspection through the electron microscope are repeated until errors fall within a tolerable range.
As can be seen, a number of steps are required to carry out the above process. Since the exposure device cannot be used during the above process, the utilization of the exposure device is reduced. Also, correction amounts are obtained by educated guess (trial and error), an adjustment of exposure device in a short time is difficult.
Accordingly, there is a need for a charged-particle-beam exposure device and a charged-particle-beam exposure method in which the cross-sectional shape of an electron beam can be checked without the pattern-check steps to provide appropriate correction amounts.
In summary, there are needs in the field of the charged-particle-beam exposure technique for a device and a method in which the cross-sectional shape of the electron beam can be checked.
In the block-type exposure technique, there is a need for a charged-particle-beam exposure device and a charged-particle-beam exposure method which allow mask-pattern inspection in order to prevent pattern-exposure deficiency on a wafer from developing due to mask deficiency or the like.
In the variable-rectangle exposure technique, there is a need for a charged-particle-beam exposure device and a charged-particle-beam exposure method in which the cross-sectional shape of an electron beam can be checked without the pattern-check steps to provide appropriate correction amounts.